1. Field of the Invention
The present invention generally relates to a direct conversion receiving apparatus and a cellular phone, and particurally, to a wireless direct conversion receiving apparatus provided with a structure for supressing transitional DC offset variation when the gain of a gain control amplifier is switched, and a cellular phone using the same.
2. Description of the Related Art
In recent years, in a receiving apparatus for wireless communication such as a cellular phone, miniaturization and a low cost have been highly required. As a system capable of decreasing the number of parts, a direct conversion system has been known. The direct conversion system is a system to multiply a high frequency (RF) signal received by an antennal by a local oscillation signal (a local signal) of the substantially same frequency as the high frequency and directly take out a base band signal while omitting conversion into a middle frequency. By employing the direct conversion system, a monolithic IC can be realized.
However, the direct conversion system has defects, and one of them relates to a DC (direct current) offset variation. Since the DC offset variation causes deterioration of a BER (bit error rate) and decrease of a dynamic range of an A/D converter, or the like, a method to suppress the DC offset variation has been conventionally proposed.
As the simplest example of the above-described method, there is a method of inserting a high pass filter that is set at a cutoff frequency for minimizing the deterioration of the BER into a direct conversion receiving apparatus. However, only by inserting the high pass filter, a static DC offset variation can be suppressed, however, a transitional DC offset variation when the gain of a gain control amplifier is switched cannot be suppressed. Therefore, a method of suppressing the transitional DC offset variation is very important.
The direct conversion receiving apparatus according to a conventional example using a method of suppressing the transitional DC offset variation is disclosed in JP-A-2003-224489. With reference to FIG. 7 and FIG. 8, the direct conversion receiving apparatus according to the conventional example will be described below. FIG. 7 shows a structure of the direct conversion receiving apparatus according to the conventional example.
The conventional direct conversion receiving apparatus includes an input terminal 101, an RF amplifier 102, a VCO (local oscillator) 103, a 0°/90° phase converter 104, a mixer 105, gain control amplifiers (GCA) 106a, 106b, 106c, low pass filters (LPF) 107a, 107b, 107c, high pass filters (HPF) 119a, 119b, 119c, a control circuit 708, an all pass filter 115, a buffer amplifier 116, and an output terminal 117.
The gain control amplifiers (GCA) 106b and 106c are configured as same as the GCA 106a, and the low pass filters (LPF) 107b and 107c are configured as same as the LPF 107a. 
In the conventional wireless direct conversion receiving apparatus, the high pass filter (HPF) 119a has first capacitor 109a and 109b which are connected between low pass filter (LPF) 107a and gain control amplifiers (GCA) 106b, first resistor 112a and 112b which are connected between a reference voltage (Vref) and gain control amplifiers 106b, and second time constant switching circuits 113a and 113b which are connected to the first resistor in parallel. The high pass filters (HPF) 119b and 119c are configured as same as the high pass filter 119a. 
In the conventional wireless direct conversion receiving apparatus, a RF (radio frequency) signal received in an LNA (a low noise amplifier) and a SAW filter (not illustrated) through an antenna is input in the input terminal 101 of an IC for the RF. The RF amplifier 102 amplifies the RF signal input in the input terminal 101 and outputs it.
The VCO 103 outputs a local signal of a local oscillation frequency. The 0°/90° phase converter 104 divides the frequency of the local signal into four and outputs it. The local signal having its frequency divided into four has the same frequency as the center frequency of the RF signal input in the input terminal 101.
The mixer 105 inputs the RF signal and the local signal, demodulates them in orthogonal, generates a base band signal from the RF signal, and output it.
The gain control amplifier (GCA) 106a inputs a base band signal output from the mixer 105 and, then, amplifies the base band signal with a gain decided based on the gain switching control signal of a control circuit 708 and outputs it.
The low pass filter (LPF) 107a inputs the baseband signal, cuts off the high band frequency, and outputs this signal.
The high pass filter (HPF) 119a inputs the base band signal output from the LPF 107a, cuts off the low band frequency through the second time constant switching circuit 113a controlled by the capacitor 109a for cutting DC, the resistor 112a, and the control circuit 708 and outputs it.
The signal output from the high pass filter (HPF) 119a passes through the GCA 106b, the LPF 107b, and the HPF 119b at a second stage, and the GCA 106c, the LPF 107c, and the HPF 119c at a third stage.
The control circuit 708 outputs the gain switching control signal to the gain control amplifiers (GCA) 106a, 106b, and 106c, and outputs the second time constant switching control signal to the high pass filters 119a, 119b, and 119c. 
The all pass filter (APF) 115 makes the phase of the base band signal output from the LPF 107c flat to output it.
The base band signal is output from the output terminal 117 to the outside via the buffer amplifier 116.
FIG. 8 is a wave form chart explaining the operation of the conventional direct conversion receiving apparatus. While inputting the RF signal as shown in FIG. 8A, as shown by the wave from of FIG. 8B, the gain of the gain control amplifier 106a is switched from gain 1 into gain 2 at timing of a time t2.
As shown in FIG. 8C, the second time constant switching circuit 113a is turned on only for a predetermined period (t2 to t4) by the control of the control circuit 708 to make the cutoff frequency of the high pass filter 119a higher (for example, about 1 MHz). Thereby, the conventional direct conversion receiving apparatus can suppress the transitional DC offset that is generated when switching the gain of the time t2.
According to the conventional direct conversion receiving apparatus, when the gain change of the GCA is sufficiently small, turning off the second time constant switching circuit 113a, the cutoff frequency of the HPF is made lower as much as possible. On the contrary, when the gain change excesses a predetermined value, the second time constant switching circuit 113a is turned on for a predetermined period to make the cutoff frequency of the HPF higher for this period, suppressing the transitional DC offset variation of the gain switching time (t2) of the GCA.
According to the conventional direct conversion receiving apparatus, at a time t4 when the second time constant switching circuits 113a and 113b are turned off from on, when the input signal into the capacitor 109 of the high pass filter 119a is located at a bottom of the wave form, for example, as shown in FIG. 8A, there is a problem such that the output signal of the high pass filter 119a generates the transitional DC offset at the time t4 as shown in FIG. 8D.
On and after the time t4, the time constant of the high pass filter 119a is set so as to prevent the wave form of the normal base band signal from being deteriorated (for example, 10 kHz), so that a period of (t4 to t7) is required in order for the transitional DC offset generated at the time t4 to converge. Since this period is very long, the BER is deteriorated. In the same way, also in the gain control amplifiers 106b and 106c at the rear stage, the transitional DC offset variation is caused when switching the gain.